The development of highly absorbent articles for use as disposable diapers, adult incontinence pads and briefs, and sanitary napkins, are the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging and reduced weight also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain particulate absorbent polymers often referred to as "hydrogels," "superabsorbents" or "hydrocolloid" materials has been particularly important. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such particulate absorbent polymers in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent scores that take advantage of the ability of these particulate absorbent polymers to absorb large quantities of discharged aqueous body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et at), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and particulate absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
These particulate absorbent polymers were previously unsurpassed in their ability to retain large volumes of fluids, such as urine. A representative example of such particulate absorbent polymers are lightly crosslinked polyacrylates. Like many of the other absorbent polymers, these lightly crosslinked polyacrylates comprise a multiplicity of anionic (charged) carboxy groups attached to the polymer backbone. It is these charged carboxy groups that enable the polymer to absorb aqueous body fluids as the result of osmotic forces.
Absorbency based on capillary forces is also important in many absorbent articles, including diapers. Capillary forces are notable in various everyday phenomena, as exemplified by a paper towel soaking up spilled liquids. Capillary absorbents can offer superior performance in terms of the rate of fluid acquisition and wicking, i.e., the ability to move aqueous fluid away from the point of initial contact. Indeed, the dual-layer core absorbent structures noted above use the fibrous matrix as the primary capillary transport vehicle to move the initially acquired aqueous body fluid throughout the absorbent core so that it can be absorbed and retained by the particulate absorbent polymer positioned in layers or zones of the core.
Other absorbent materials capable of providing capillary fluid transport are open-celled polymeric foams. Indeed, certain types of polymeric foams have been used in absorbent articles for the purpose of actually imbibing, wicking and/or retaining aqueous body fluids. See, for example, U.S. Pat. No. 3,563,243 (Lindquist), issued Feb. 6, 1971 (absorbent pad for diapers and the like where the primary absorbent is a hydrophilic polyurethane foam sheet); U.S. Pat. No. 4,554,297 (Dabi), issued Nov. 19, 1985 (body fluid absorbing cellular polymers that can be used in diapers or catamenial products); U.S. Pat. No. 4,740,520 (Garvey et al), issued Apr. 26, 1988 (absorbent composite structure such as diapers, feminine care products and the like that contain sponge absorbents made from certain types of super-wicking, crosslinked polyurethane foams).
The use of absorbent foams in absorbent articles such as diapers can be highly desirable. If made appropriately, open-celled hydrophilic polymeric foams can provide features of capillary fluid acquisition, transport and storage required for use in high performance absorbent cores. Absorbent articles containing such foams can possess desirable wet integrity, can provide suitable fit throughout the entire period the article is worn, and can minimize changes in shape during use (e.g., swelling, bunching). In addition, absorbent articles containing such foam structures can be easier to manufacture on a commercial scale. For example, absorbent diaper cores can simply be stamped out of continuous foam sheets and can be designed to have considerably greater integrity and uniformity than absorbent fibrous webs. Many absorbent cores made from such fibrous webs fall apart during use. Such foams can also be molded into any desired shape, or even formed into integral, unitary diapers.
Particularly suitable absorbent foams for absorbent products such as diapers have been made from High Internal Phase Emulsions (hereafter referred to as "HIPEs"). See, for example, U.S. Pat. No. 5,260,345 (DesMarais et at), issued Nov. 9, 1993 and U.S. Pat. No. 5,268,224 (DesMarais et at), issued Dec. 7, 1993. These absorbent HIPE foams provide desirable fluid handling properties, including: (a) relatively good wicking and fluid distribution characteristics to transport the imbibed urine or other body fluid away from the initial impingement zone and into the unused balance of the foam structure to allow for subsequent gushes of fluid to be accommodated; and (b) a relatively high storage capacity with a relatively high fluid capacity under load, i.e., under compressive forces. These HIPE absorbent foams are also sufficiently flexible and soft so as to provide a high degree of comfort to the wearer of the absorbent article, and can be made relatively thin until subsequently wetted by the absorbed body fluid.
An important issue in making absorbent HIPE foams commercially attractive for use in absorbent products such as diapers is economics. The economics of absorbent HIPE foams depend on the amount and cost of the monomers used per unit of fluid absorbed, as well as the cost of convening the monomers to a usable polymeric foam. Making absorbent HIPE foams economically attractive can require using: (1) less total monomer per unit volume of foam: (2) less expensive monomers: (3) a less expensive process for converting these monomers to a usable absorbent HIPE foam; or (4) combinations of these factors. At the same time, the absorbent HIPE foam must satisfy desired characteristics for absorbent capacity and strength under compressive load without sacrificing tear resistance or resilience to an unacceptable degree. The effort to reduce the cost of such absorbent foams, especially in terms of reducing the total amount of monomer used, can make it very difficult to achieve these desired absorbency and mechanical properties.
As previously noted, a thinner absorbent core is usually a requirement for making relatively thin absorbent articles, such as diapers. Providing relatively thin absorbent HIPE foams that rapidly absorb body fluids when wetted can be very challenging. This especially true if the relatively thin HIPE foam is to be made economically, while at the same time satisfying the desired criteria for absorbent capacity, toughness and strength under compressive load. For example, it has been found that when less monomer is used per unit volume, the resulting absorbent HIPE foam can be too weak to function properly.
Accordingly, it would be desirable to be able to make an open-celled absorbent polymeric foam material that: (1) has adequate or preferably superior fluid handling characteristics, including capillary fluid transport capability and total absorbent capacity for discharged body fluids so as to be desirable for use in absorbent articles such as diapers, adult incontinence pads or briefs, sanitary napkins and the like; (2) can be relatively thin and lightweight during normal storage and use until wetted with these body fluids; (3) has sufficient resiliency, toughness and strength under compressive load to rapidly absorb these body fluids; and (4) can be manufactured economically without sacrificing these desired absorbency and mechanical properties to an unacceptable degree.